This invention pertains to the problem of advancing two or more hydraulic cylinders to move a single load, such that all cylinders advance at the same rate.
Synchronized movement of pairs of hydraulic cylinders is critical in certain aircraft applications. For example, aircraft flap systems are generally segmented and each segment is driven by two or more hydraulic actuators, and if the actuators are not synchronized in advance and retract the surfaces are subjected to undue stress. In some cases non-synchronization may cause the system to bind. Also the segments of the flap system must advance at the same rate. The same is true of large cargo doors. Thrust reversers in jet aircraft which retard the forward momentum of the aircraft after landing are generally operated by multiple hydraulic actuators. Thrust reverser movement must be synchronized although the loads in the individual actuators may vary. The problem, of course, is not limited to aircraft.
Synchronized movement of multiple hydraulic cylinders may be achieved by metered control of fluid flow, e.g. flow dividers and split delivery pumps. These methods must be made very complex in order to meet synchronization accuracies required. A more novel approach which inherently lends itself to the accuracies required was taught in U.S. Pat. No. 3,855,794 to Meyer et al and in an article which appeared in a periodical entitled HYDRAULICS AND PNEUMATICS published by Penton/IPC, Inc., Cleveland, Ohio in the February 1974 issue at page 39. Both of these disclosures teach using the rearward piston and that portion of the cylinder which, of course, produces two cavities which are used for the power stroke to extend and retract the piston while the front two chambers are used to synchronize the two cylinders. The two cavities of the respective synchronization cylinders are cross plumbed i.e. the front cavity of the first cylinder is plumbed to the rear cavity of the second cylinder and the second cavity of the first cylinder is plumbed to the first cavity of the second cylinder. The two cylinders must advance and retract together because of their interconnection. Of course, both cylinders must have the same bore and rod size so as to displace equal volumes of fluid for equal stroke. The problem with the apparatus of this teaching is that it requires a very long actuator and in many aircraft applications this length is simply not available.
Another article appeared in the same periodical, HYDRAULICS AND PNEUMATICS, in the July 1979 issue, at page 14, which disclosed a novel method of reducing the length of the actuator. However, one cylinder extends while the other cylinder retracts, both at the same speed, of course, and then teaches connecting the rod ends to different mechanical linkages with reversed motion. This approach may also create a space problem at the actuator end.
Another approach for synchronizing cylinders was taught in U.S. Pat. No. 4,241,581 to Chase which discloses a synchronizer which consists of a two piston pump. While this teaching does permit the two-piston pump to be located remotely from the actuator it requires a much more complex circuit, it does not inherently lend itself to dimensional control, and does not provide for any means to realign the actuator in case of internal leakage.
In summary, the teachings of the four references discussed in detail have their advantages and disadvantages. However, none of the art alone or in combination teaches how to solve the undue length of the actuator as taught in the first two references while retaining the inherent ease of maintaining the critical dimensions. None of the art teaches adequate means for adjusting the synchronization of the two cylinders because of leakage.